WO2023161113A1 - Mems tapping-mode cantilever as acoustic nanoforce sensor - Google Patents

Abstract

In a first aspect, the invention relates to a MEMS microphone for detecting acoustic signals. The MEMS microphone has a vibratable microphone diaphragm which is excited to vibrate by sound waves which pass through a sound entry opening. The MEMS microphone also has a cantilever comprising a measurement tip. The cantilever is actively excited to vibrate by an actuator, with the result that the measurement tip is guided in a vibrating manner to the microphone diaphragm without contact. In this case, a tunnel current flows between the microphone diaphragm and the measurement tip and can be used to detect the vibration behaviour of the microphone diaphragm that depends on the sound waves. An electronic circuit is configured to measure the tunnel current between the microphone diaphragm and the measurement tip. The MEMS microphone according to the invention makes it possible to detect particularly low sound pressure levels with high resolution. In a further aspect, the invention relates to a method for detecting acoustic signals using the MEMS microphone according to the invention.

Description

MEMS TAPPING MODE CANTILEVER AS ACOUSTIC NANO FORCE SENSOR

DESCRIPTION

The invention relates to the technical field of MEMS microphones. In a first aspect, the invention relates to a MEMS microphone for detecting acoustic signals. The MEMS microphone has an oscillatable microphone membrane that is excited to vibrate by sound waves that pass through a sound inlet opening. Furthermore, the MEMS microphone has a cantilever including a measuring tip. The cantilever is actively excited to oscillate by an actuator, so that the measuring tip is guided to the microphone membrane in a non-contact oscillating manner. A tunnel current flows between the microphone membrane and the measuring tip, with which the vibration behavior of the microphone membrane, which depends on the sound waves, can be detected. An electronic circuit is set up to measure the tunneling current between the microphone membrane and the measuring tip. The MEMS microphone according to the invention makes it possible to detect particularly low sound pressure levels with high resolution.

In a further aspect, the invention relates to methods for detecting acoustic signals using the MEMS microphone according to the invention.

Background and prior art

Microphones are electro-acoustic transducers that convert a sound event, i. H. convert sound waves into an electrical signal. The electrical signal corresponds to the acoustic input signal. In the state of the art, there is a large number of designs and functionalities of different types of microphones, each of which fulfills different application purposes and functions.

In particular, extremely compact microphones based on MEMS technology, so-called MEMS microphones, are known in the prior art. MEMS is the abbreviation for microelectromechanical system (microelectromechanical system) and is characterized by a miniaturized and compact design, which is particularly present in the micrometer range and has excellent functionality with ever lower manufacturing costs.

A MEMS microphone usually includes an oscillatable microphone membrane that is set up to record pressure waves of a fluid. The fluid can be either a gaseous or a liquid fluid, preferably sound pressure waves. A MEMS microphone preferably converts pressure waves into electrical signals and thus represents a sound detector.

MEMS microphones have a number of advantages. For example, due to their compact design, they can be arranged in arrays in a particularly simple manner, which can be used for sound measurements with a directional characteristic. In addition, they can be manufactured using common, largely automated processes in semiconductor technology. It is known from the prior art to combine MEMS microphones with other measurement sensors in order to measure a number of measurement signals and thereby be able to cover more diverse measurement ranges and applications. For example, US 2015/0158722 A1 discloses a MEMS device which has a MEMS microphone and a further MEMS sensor in the form of a motion sensor. In an embodiment described therein, the membrane of the MEMS microphone is in front of a perforated backplate. The MEMS microphone sits on a substrate that has an opening for sound waves to enter. Furthermore, there is another sensor on the substrate, which is z. B. can be a gyroscope, an acceleration sensor or a pressure sensor. In addition to the measurement of quantities by the MEMS microphone and the other sensors, other information can also be measured and/or processed, such as ultrasonic waves, infrared light, temperature, humidity and/or a gas species in the environment of the MEMS device.

However, the majority of MEMS microphones are designed for audio applications; H. for phones and/or hearing aids. These applications are mostly characterized by a bandwidth of less than 20 kHz and a sound pressure level of less than approx. 120 dB.

In the prior art, however, there is also an attempt to measure particularly low sound pressure levels, in particular using methods from microsystems technology. This is relevant, for example, in photoacoustic spectroscopy. In photoacoustic spectroscopy, intensity-modulated radiation with frequencies in the absorption spectrum of a molecule to be detected in a gas is used. If this molecule is present in the beam path, modulated absorption takes place, leading to heating and cooling processes whose time scales reflect the modulation frequency of the radiation. The heating and cooling processes cause the gas to expand and contract, causing sound waves at the modulation frequency. These can be measured, for example by sound detectors such as MEMS microphones. The sound pressures that occur are very low.

Photoacoustic gas sensors as such are already well known in the prior art. A variant of a photoacoustic gas sensor is disclosed in US 2011/0296900 A1. All components for operational suitability are installed in one or as a MEMS device. The photoacoustic gas sensor has an infrared source mounted on a substrate. An integrated microphone can be on the substrate itself or on a second substrate. A filter for the infrared light is present on a third substrate.

Approaches are also known in the prior art for measuring low sound pressure levels, in particular for the purposes of photoacoustic spectroscopy. In photoacoustic spectroscopy in particular, the measurement of low sound pressure levels is relevant in order to be able to record reliable measurement results.

In Sievilä et al. (2013) discloses a manufacturing method of a cantilever and a sensor arrangement to measure the sound pressure level in the context of photoacoustic spectroscopy. In this case, modulated infrared light is emitted into a sample cell containing a gas that absorbs the wavelength of the infrared light. At one end of the A cantilever is located in the sample cell, which is deflected by the absorption of the infrared light. Outside the sample cell is a Michelson-Morley interferometer. The beam path of the Michelson-Morley interferometer is designed in such a way that it hits the cantilever through a transparent window. A change in the absorption of the gas also affects the position of the cantilever and thus also the beam path of the Michelson-Morley interferometer. A change in the sound pressure is thus measured. Further applications are not described here. The structure is complex, particularly due to the use of a Michelson-Morley interferometer, and is disadvantageous for a compact configuration of the measuring system.

With the MEMS microphones known in the prior art, it is also only possible to a limited extent to precisely measure low sound pressure levels. In particular, most measurements with MEMS microphones are unsuitable for low frequencies and only have a low signal-to-noise ratio in this range.

US 2005/0249041 A1 discloses a MEMS microphone that is said to have increased sensitivity. The MEMS microphone of US 2005/0249041 A1 is intended i.a. have various advantages over a theoretical approach outlined in the introductory paragraphs [0007] and [0008] of US 2005/0249041 A1. According to the approach discussed, a tunnel current between a measuring tip and the membrane is kept constant by a closed control loop. For this purpose, the cantilever, to which the measuring tip is attached, is tracked when the membrane is caused to vibrate by impacting sound waves. According to the teaching US 2005/0249041 A1, such a method would disadvantageously have a high sensitivity against B. vibration and be more expensive to manufacture. In addition, according to US 2005/0249041 A1, with such an approach it can happen that the resonant frequency of the cantilever falls within the range of frequencies to be detected, making it difficult if not impossible to check the measuring tip. In this respect, such an approach according to the disclosure of US 2005/0249041 A1 is unsuitable for providing a MEMS microphone.

In contrast, the MEMS microphone disclosed according to the teaching of US 2005/0249041 A1 comprises a measuring tip which is located on a perforated, ie provided with openings, support plate and directly behind the microphone membrane. The microphone membrane is caused to vibrate by the sound waves that occur. A tunnel current flows between the measuring tip on the support plate and the microphone membrane, which is evaluated as a measuring signal. The measuring tip itself does not move, which is intended to reduce the influence of vibration and eliminate the need to check the movement of the measuring tip. However, the sound to be detected can be lost through the openings in the support plate, with the result that the sound is not measured with great accuracy. Furthermore, if high sound pressures occur, the microphone membrane can touch the measuring tip, since the measuring tip is not designed to be movable. The measurement is thus static. If the measuring tip and the microphone membrane touch, a tunnel current no longer flows, but an ohmic current, which leads to a change in the measurement signal and can affect reliable and long-term operation. There is therefore a need for improvement with regard to the provision of MEMS microphones for sensitive measurements, even at low sound pressure levels.

object of the invention

The object of the invention was to eliminate the disadvantages of the prior art and to provide a MEMS microphone with which low sound pressure levels can be measured with high resolution. Furthermore, the MEMS microphone should preferably be characterized by the possibility of particularly reliable and long-lasting measurements and an inexpensive and compact structure.

Summary of the Invention

The object according to the invention is solved by the features of the independent claims. Advantageous configurations of the invention are described in the dependent claims.

In a preferred embodiment, the invention relates to a MEMS microphone for detecting acoustic signals, comprising a sound entry opening, an oscillatable microphone membrane and an electronic circuit, with sound waves entering through the sound entry opening exciting the oscillatable microphone membrane to oscillate, characterized in that the MEMS microphone has a cantilever comprising a measuring tip and an actuator, wherein the electronic circuit is set up to measure a tunnel current between the microphone membrane and the measuring tip and to actively excite the cantilever to oscillate, with the measuring tip being guided to the microphone membrane in a contactless oscillating manner in order to detect acoustic signals, while the measurable tunnel current allows detection of the vibration behavior of the microphone membrane, which is dependent on the sound waves.

The MEMS microphone according to the invention has proven to be extremely advantageous in that it can measure low sound pressure levels precisely with a particularly high resolution. The MEMS microphone has a significantly higher sensitivity than the known MEMS microphones of the prior art. This is noticeable, for example, in the fact that a particularly high signal-to-noise ratio can be achieved despite the low sound pressure level.

The high sensitivity at low sound pressure levels is based on the advantageous measuring principle using a tunnel current. The cantilever is preferably guided to the microphone diaphragm, encompassing the measuring tip, by the actuator and vibrating without contact. The distance between the measuring tip and the microphone membrane is so small that a tunnel current flows between the microphone membrane and the measuring tip due to the quantum mechanical tunnel effect. The tunnel current preferably flows by applying a voltage. The tunnel current depends exponentially on the distance between the microphone membrane and the measuring tip. Due to the highly sensitive dependence of the tunnel current on the distance, even the smallest changes in distance between the microphone membrane and the measuring tip can be recorded by measuring the tunnel current. Through the active excitation of the cantilever (in particular the measuring tip) to oscillate, the distance between the measuring tip and the microphone membrane preferably changes periodically with the known periodicity of the exciting oscillation. The tunnel current is therefore preferably a periodic signal whose amplitude depends on the distance between the measuring tip and the microphone membrane during the oscillation.

If a sound event occurs, i. H. If sound waves hit the microphone membrane, the microphone membrane is deflected and itself set into vibration. This changes the distance between the microphone membrane and the vibrating measuring tip. The measurable tunnel current therefore changes as a function of the vibration behavior of the microphone diaphragm, which is dependent on the sound waves.

To detect acoustic signals by measuring the tunnel current, the device (explained in detail later) can be configured differently.

For example, analogous to a constant height mode, the detection can be carried out in such a way that the deflection and center position of the oscillating measuring tip on the cantilever is not changed, so that the distance between this and the measuring tip changes due to a vibration of the microphone membrane and thus also the tunnel current.

Alternatively, for example, the device can also be designed such that the center position of the oscillation of the cantilever is adjusted in order to keep the amplitude of the tunnel current constant. This configuration would correspond to a constant current mode measurement principle, with the tunnel current or its amplitude being used as a controlled variable for the middle position of the oscillating measuring tip.

In any case, according to the invention, advantage is taken of the fact that the tunnel current signal is extremely sensitive even to the smallest deflections of the microphone diaphragm, so that extremely small changes in the sound pressure level are reliably detected.

Particularly low sound pressure levels can advantageously be measured with high accuracy by means of the device according to the invention. In this way, the MEMS microphone according to the invention can advantageously measure sound pressure levels which, in particular, are less than approx.

20 dB (decibels), preferably less than about 10 dB, more preferably less than about 5 dB, and even less than about 1 dB.

It is particularly advantageous that the MEMS microphone according to the invention can measure low sound pressure levels with a particularly high signal-to-noise ratio.

Terms such as essentially, approximately etc. preferably describe a tolerance range of less than ±40%, preferably less than ±20%, particularly preferably less than ±10%, even more preferably less than ±5% and in particular less than ±1% and include in particular the exact value. Partially describes preferably at least 5%, more preferably at least 10%, and especially at least 20%, in some cases at least 40%.

Furthermore, the method according to the invention is advantageous in that contact with the microphone membrane is avoided due to the ability of the measuring tip to move, in particular due to the vibration of the measuring tip. A possible touch could for example, in US 2005/0249041 A1, when the microphone membrane experiences strong deflections at high sound pressure levels. Advantageously, this can be avoided by the MEMS microphone according to the invention, since the measuring tip is guided to the microphone membrane in a non-contact oscillating manner. Consequently, wear and tear is avoided and long-lasting functionality of the MEMS microphone according to the invention is ensured.

Furthermore, the dynamic measurement of the tunnel current by means of a non-contact oscillating measuring tip allows highly sensitive scanning of the vibration behavior of the membrane for almost any frequency range. In particular, the modulation frequency of the oscillating measuring tip can be freely selected, so that measurements can be taken at very low (0 Hz) to very high frequencies (MHz) regardless of the excitation frequency. Because the modulation frequency of the measuring tip is independent of the excitation frequency, a high signal-to-noise ratio can advantageously be achieved over a wide frequency range. This represents a particular advantage over known MEMS microphones, which exhibit increased noise, particularly at lower frequencies.

In the context of the invention, it is preferably provided that the cantilever, including the measuring tip, performs a periodic oscillation, to which it is actively excited. The periodic oscillation of the cantilever preferably takes place in the sense of a tapping. This preferably means that the cantilever executes a continuous, periodic oscillation that is independent of a movement of the microphone membrane. Instead, the measuring tip is guided to the microphone membrane while vibrating without contact, in order to enable the detection of any vibrations of the membrane if it is excited by sound waves. With such a contactless oscillation of the cantilever in the sense of a tapping, the measuring tip consequently changes the distance to the oscillating membrane, preferably periodically.

As a result, the MEMS microphone according to the invention also differs from the disadvantageous approach described in US 2005/0249041 A1 in paragraphs [0007] and [0008], in which a measuring tip of a cantilever tracks any vibrations of the membrane. In the approach described, there is currently no active excitation of a cantilever, which is guided to the microphone membrane while vibrating without contact. Instead, the distance between the measuring tip and the membrane is kept constant.

The actively excited periodic oscillation of the cantilever in the sense of tapping according to the invention is therefore not to be compared with tracking the cantilever to oscillations of the microphone membrane, which merely follows any movements of the membrane due to sound excitation. Instead, the actively stimulated periodic oscillation of the cantilever occurs independently of the oscillations of the microphone membrane and is maintained over the entire detection process.

This is also made clear by the fact that the vibration of the cantilever preferably takes place at a frequency which is many times higher (for example by a factor of 2, 3, 4 or more) than the expected vibrations of the microphone membrane. In contrast to tracking the cantilever, a disadvantageously high sensitivity with regard to vibrations can also be avoided. Active excitation of the cantilever to cause periodic oscillations can thus advantageously ensure the measurement of sound events with high sensitivity even at an extremely low sound pressure level. The MEMS microphone according to the invention advantageously succeeds in reducing or eliminating the influence of interference factors.

In addition, the MEMS microphone according to the invention is advantageous in that there are no flow losses in the fluid in which the sound propagates. Instead, the sound preferably hits the microphone membrane directly, without the sound waves being distorted or energy losses being caused by other components. A particularly accurate and undistorted signal can thus advantageously be measured. In this regard, it can also be preferred that the MEMS microphone according to the invention has no further openings apart from a sound entry opening.

In addition, the MEMS microphone according to the invention can advantageously be produced in a particularly process-efficient manner, since it can be provided using standardized processes in semiconductor and microsystems technology. Here, in particular, proven, automated methods of semiconductor processing can be used in order to implement cost-effective mass production.

The MEMS microphone according to the invention uses a new application of measuring tips that are known from scanning tunneling microscopy and can therefore also use known processes for producing such measuring tips. However, the measuring tip according to the invention is not used to record the structure of a material surface, but rather for acoustic purposes to detect sound events. The MEMS microphone according to the invention thus efficiently combines two different technical fields in order to enable a significant improvement in terms of the detection of acoustic signals.

The MEMS microphone according to the invention represents an advantageous further development of conventional MEMS microphones, in which highly sensitive measurements of the vibration behavior of a microphone membrane are used by dynamic measurement of a tunnel current and the measurement range of the MEMS microphone in terms of sound pressure level can be extended well below 20 dB . As explained above, the fact that the distance between the measuring tip and the microphone membrane depends exponentially on the distance is used here. Alternative measurement methods, such as capacitive measurement methods, only show a linear correlation between the (capacitive) measurement signal and the deflection of the microphone membrane. While a MEMS microphone based on a tunnel current is less interesting for classic audio applications due to the non-linearity, it was recognized according to the invention that the non-linearity can be exploited for highly sensitive sound detections, for example for photoacoustic spectroscopy.

A MEMS microphone preferably designates a microphone which is based on MEMS technology and whose sound-recording structures are at least partially dimensioned in the micrometer range (approx. 1 μm to approximately 1000 μm). The sound-recording structures are preferably referred to as microphone membranes. The width, height and/or thickness of the microphone membrane can preferably be less than 1000 μm. The microphone diaphragm is preferably two-dimensional, which means in particular that its extension in each of the two dimensions (height, width) of its surface is greater than in one dimension perpendicular thereto (the thickness). For example, size ratios of at least 5:1, preferably at least 10:1, 50:1 or more can be preferred.

In a preferred embodiment, the length or width of the microphone membrane is between 1 μm and 1000 μm, preferably between 10 μm and 500 μm. Intermediate ranges from the above ranges can also be preferred, such as 1 μm to 10 μm, 10 μm to 50 μm, 50 μm to 100 μm, 100 μm to 200 μm, 200 μm to 300 μm, 300 μm to 400 μm, 400 μm to 500 μm pm, 600 pm to 700 pm, 700 pm to 800 pm, 800 pm to 900 pm or also 900 pm to 1000 pm. A person skilled in the art recognizes that the aforementioned range limits can also be combined in order to obtain further preferred ranges, such as 10 μm to 200 μm, 50 μm to 300 μm or also 100 μm to 600 μm.

In a preferred embodiment, the thickness of the membrane is between 100 nm and 10 μm, preferably between 500 nm and 5 μm. Intermediate ranges from the aforementioned ranges can also be preferred, for example 100 nm to 500 nm, 500 nm to 1 μm, 1 μm to 1.5 μm, 1.5 μm to 2 μm, 2 μm to 3 μm, 3 μm to 4 μm , 4 pm to 5 pm, 5 pm to 6 pm, 6 pm to 7 pm, 7 pm to 8 pm, 8 pm to 9 pm or also 9 pm to 10 pm. A person skilled in the art recognizes that the aforementioned range limits can also be combined in order to obtain further preferred ranges, such as 500 nm to 3 pm, 1 pm to 5 pm or even 1500 nm to 6 pm.

In the course of the development of MEMS microphones, different construction types have become established. These can be categorized according to the type of sound they are fed into. If the sound hits the microphone membrane via the underside of the housing, this is referred to as a bottom-port MEMS microphone. Bottom-port MEMS microphones also require an opening on the substrate on which the components of the MEMS microphone are located (also called the carrier substrate), because this is the only way for sound waves to find their way to the microphone membrane. If the sound hits the sensor via the top of the housing, it is called a top-port MEMS microphone. Whether a top-port or bottom-port MEMS microphone is preferred mostly depends on factors such as the placement of the microphone in the product and/or manufacturing considerations. The MEMS microphone can be a top port MEMS microphone or a bottom port MEMS microphone.

The microphone membrane is set up to record pressure waves of the fluid. The fluid can be either a gaseous or a liquid fluid, preferably sound pressure waves. A MEMS microphone therefore preferably converts pressure waves into electrical signals. The microphone membrane is preferably sufficiently thin so that it bends under the influence of the changes in air pressure caused by the sound waves and changes to an oscillating behavior. As the microphone membrane vibrates, electrical quantities can change, such as the current strength of the tunnel current between the microphone membrane and the measuring tip. The change in an electrical variable can be measured, recorded and/or evaluated by an electronic circuit preferably installed in the MEMS microphone, for example an ASIC or a computing unit. The electronic circuit preferably measures changes in such electrical quantities that arise when the microphone membrane vibrates under the influence of sound waves. The electronic circuit preferably converts the vibrations of the microphone diaphragm into electrical signals. The electronic circuit preferably includes electrical connections, for example wires. The electronic circuit is preferably connected to the cantilever and/or the measuring tip, so that the tunnel current can preferably be read out.

In addition, the electronic circuit can have an ASIC (English application-specific integrated circuit, German application-specific integrated circuit), a computing unit, an integrated circuit (IC), a programmable logic circuit (PLD), a Field Programmable Gate Array (FPGA), a microprocessor , Have a microcomputer, a programmable logic controller and / or other electronic circuit elements.

The electronic circuit is preferably set up on the one hand to carry out a measurement of a tunnel current between the microphone membrane and the measuring tip. On the other hand, the electronic circuit is preferably set up to bring about an active excitation of the cantilever to oscillate.

The formulation according to which the electronic circuit is set up to carry out a specific method step, such as measuring a tunnel current or actively exciting the cantilever to oscillate, preferably means that software or firmware is installed on the electronic circuit, which includes the commands mentioned carry out procedural steps.

In addition, the electronic circuit is preferably programmable, so that settings with regard to the operation of the MEMS microphone can have different operating options. It can thus be preferred that an excitation mode with regard to the measuring tip and/or the cantilever can be selected, for which purpose various operating options can be installed in the software, for example.

The sound entry opening preferably designates an opening of the MEMS microphone through which sound waves can pass and impinge on the microphone membrane. The sound entry opening is preferably located in the flow direction of the fluid, in particular air, in front of the microphone membrane.

An acoustic signal preferably designates an electrical signal that is produced by sound. Sound is preferably a mechanical deformation in a medium that progresses as a wave. In a fluid, sound is always a longitudinal wave, especially in air. The terms “sound” and “sound wave” can therefore be used synonymously. In gases such as air, sound can be described as a sound pressure wave superimposed on the static air pressure. In the case of sound waves, the fluctuations in the state variables pressure and density are usually small in relation to their rest variables. If air is discussed below as the fluid of the sound waves, the person skilled in the art knows that the explanations can also be transferred to other fluids.

A cantilever, or synonymously a bending beam, is preferably a spatially extended, in particular elongate, element which is mounted so as to be able to oscillate along at least one side and is otherwise preferably free-standing. The oscillatable side of the cantilever can also be referred to as the free end. A cantilever can e.g. B. have the shape of a flat, elongated cuboid, the thickness of which is significantly smaller compared to the transverse and / or longitudinal extent, with the transverse extent preferably being smaller than the longitudinal extent. For example, it may be preferred that the cantilever has a thickness of 0.1 μm to 10 μm, preferably 0.5 μm to 5 μm, a length of 10 μm to 1000 μm, preferably 20 μm to 500 μm and a width of 5 pm to 100 pm, preferably 10 pm to 50 pm. In particular, the thickness of the cantilever is relevant, since greater thickness is associated with a reduced ability to bend. It is therefore preferred that the thickness is a multiple, ie a factor of 2, 3, 5, 10 or more, than the length and/or width. Furthermore, it is preferred that the width is also smaller than the length. However, a bending beam that is mounted so that it can swing on both sides or on more than one side can also be preferred. The cantilever can be in different designs, which are preferably relevant for the selection of the actuator.

In this case, the cantilever can preferably be a unimorph or monomorph cantilever, which preferably comprises an active layer and an inactive or passive layer. In this case, an active layer preferably designates a piezoelectric layer in which a force or a deformation is triggered by an applied electrical field, in particular by applying an electrical control voltage (which can be generated by the electronic circuit). This force or deformation preferably produces a bending and/or a deformation of the beam, which can preferably trigger an active oscillation by means of a periodic electrical control signal. In this case, the inactive layer preferably comprises a non-piezoelectric material. It is preferred that the active layer and the inactive layer interact in such a way that a resultant force is generated due to the control voltage applied, which causes a deflection of the beam, which preferably causes an oscillation given a periodicity of the electrical control signal. It can also be preferred that the inactive layer also comprises a piezoelectric material, which, however, is not electrically contacted and/or controlled by a control signal, to which advantageously no electrical control signal is applied and which, in particular, does not experience an external electric field that creates an internal Triggers force and/or deformation due to the indirect piezoelectric effect of the inactive layer.

Likewise, the cantilever can preferably be a bimorph cantilever, which preferably comprises at least two active layers. In this case, an inactive layer can preferably be present between the at least two active layers. It is preferred that when an electrical voltage is applied, one active layer contracts while the second active layer expands, as a result of which bending of the cantilever is advantageously achieved, which is increased in particular compared to a unimorph cantilever, ie z. B. has a larger amplitude at the same applied voltage.

The cantilever preferably includes a measuring tip. The measuring tip (English tip) is preferably located essentially at the free end of the cantilever. In this case, the electronic circuit is set up to enable the cantilever and thus also the measuring tip to oscillate in such a way that the measuring tip vibrates without contact to the To lead microphone membrane, the distance is so small that a tunnel current flows between the microphone membrane and the measuring tip. In this case, the measuring tip and the microphone membrane preferably have electrically conductive material. It is also preferred that the cantilever is made to oscillate in such a way that the measuring tip is guided to the microphone membrane in an essentially vertically oscillating manner without contact. The measuring tip preferably has only a few atomic layers in its pointed end. For example, the measuring tip can have a cross section of less than 50 nm ² , preferably less than 20 nm ² , particularly preferably less than 10 nm ² , at its pointed end.

In preferred embodiments, the measuring tip can be designed as an additional component on the cantilever and connected to the cantilever. In further preferred embodiments, the cantilever and the measuring tip are present as a common component. The measuring tip is preferably characterized in terms of its geometric shape in that the side lines meet at a common point, this common point preferably being the pointed end of the measuring tip.

A repeated fluctuation over time in the spatial deflection of the cantilever and thus in particular also of the measuring tip from a central position is preferably referred to as oscillation. In particular, the oscillation is essentially or at least partially periodic, which means above all that it is regular in terms of time. The cantilever is actively stimulated by the actuator to oscillate periodically, with the measuring tip being guided to the microphone membrane in a non-contact oscillating manner to detect acoustic signals. The vibration of the cantilever or the measuring tip is preferably carried out independently of the movements of the membrane, with the measuring tip being brought up to the membrane periodically in the sense of a tapping, without touching it. A contactless oscillation therefore preferably means that the cantilever oscillates in such a way that the measuring tip does not touch the microphone membrane during the oscillation. In particular, contact between the measuring tip and the microphone membrane is also avoided if the microphone membrane is caused to vibrate by the impact of sound waves. In particular, contact between the microphone membrane and the cantilever is avoided in order to ensure that a tunnel current is obtained and preferably to avoid a resistive current. Contact between the measuring tip and the microphone membrane in the constant height mode can be avoided, for example, in such a way that the amplitude of the cantilever and thus the measuring tip is adapted to the expected deflections of the microphone membrane. In the constant current mode, for example, contact between the measuring tip and the microphone membrane can be avoided by regulating their distance from one another using a closed loop control system in order to maintain a constant tunnel current.

Periodic oscillations, in particular viewed over a number of periods, can preferably be described by the oscillation mode of the oscillation. The vibration mode is preferably a form of description of certain temporally stationary properties of a vibration. Different vibration modes differ in particular in the spatial distribution of the vibration intensity, with the form of the vibration modes preferably being determined by boundary conditions under which the vibration propagates. These boundary conditions can B. by the material that Dimensions and / or the storage of the cantilever and preferably at least one force vector acting on the cantilever be given. The ability of the cantilever to oscillate means in particular that the cantilever can be excited to mechanically oscillate over a longer period of time by a suitable drive in the form of an actuator, without structural changes (damage) occurring.

With an oscillating cantilever it can e.g. B. give several bending vibration modes, in which the cantilever moves along a preferred direction, z. B. perpendicular to a plane of the suspension of the cantilever, which can differ in particular in the vibration frequency, the maximum vibration amplitude and its spatial occurrence. This corresponds in particular to the vibration of the measuring tip. A bending vibration mode is characterized in particular in that the vibration describes a dynamic bending process in the direction essentially of a normal to a main plane of the cantilever. Preferably, the measuring tip, which is preferably located at the free end, is periodically guided to the microphone membrane without contact by the vibration, so that a tunnel current results between the microphone membrane and the measuring tip.

In this case, in addition to the cantilever, the actuator is preferably also suitable for exciting such oscillations. In particular, the electronic circuit is operatively connected to the actuator in such a way that the actuator fulfills the function of actively exciting the cantilever. Active excitation of the cantilever preferably means active vibration excitation of the cantilever. In particular, it is preferable that the cantilever performs forced vibration by the actuator.

The actuator designates a component that converts an electrical signal, preferably originating from the electronic circuit, into a mechanical movement and/or a change in a mechanical variable. In particular, the actuator actively engages in causing the cantilever to oscillate. The actuator must be suitable for transferring a force it generates to the cantilever, e.g. B. by being connected to the cantilever in a way that enables power transmission.

Preferably, the cantilever can also at least partially encompass the actuator. The force itself must be suitable for triggering the oscillations, which means in particular that the force is periodic and preferably essentially has the frequency of the oscillations of the cantilever to be generated and is suitable for causing the cantilever and thus also the measuring tip to oscillate, preferably in a vibrational mode.

The tunnel current preferably refers to an electrical current that flows between the microphone membrane and the measuring tip, although they do not touch mechanically. In other words, the tunnel current means an electric current that flows despite a barrier between the microphone membrane and the measuring tip, the barrier meaning in particular a potential barrier that results from the gap caused by a non-existent contact between the microphone membrane and the measuring tip. In the context of the invention, the tunnel current is preferably the measured variable that allows detection of the vibration behavior of the microphone membrane that is dependent on the sound waves. It is preferably possible with the tunnel current to advantageously measure different sound quantities of the sound waves, which stimulate the microphone membrane to vibrate. Sound variables such as sound deflection, sound pressure, sound pressure level, sound energy density, sound energy, sound flow, sound velocity, sound impedance, sound intensity, sound power, sound velocity, sound amplitude and/or sound radiation pressure can be determined by measuring the tunnel current become.

The tunnel current is based on the tunnel effect, which is known from quantum mechanics or quantum physics, and is therefore a quantum mechanical or quantum physical effect and cannot be explained with the laws of classical physics. According to the laws of classical physics, a potential barrier lies, i. H. Energy barrier between the measuring tip and the microphone membrane, which prohibits the transfer of charge carriers, especially electrons. The potential barrier is particularly related to the work function.

From the point of view of quantum physics, the temporal change of the non-relativistic system is described by the Schrödinger equation. In particular, non-relativistic means that effects of the theory of relativity can be ignored. The Schrödinger equation is a partial differential equation whose solution is the wave function, which in turn describes the state of particles, in particular their location. Even in the "forbidden" range, i.e. inside and/or beyond the potential barrier, the wave function is never equal to zero, but decays exponentially there with increasing penetration depth. Even at the end of the forbidden range, their value is not zero. Since the squared magnitude of the wave function is interpreted as the probability density for the location of the particle, there is a non-zero probability for the particle to appear on the other side of the potential barrier. Since this is a quantum mechanical effect, the tunnel effect is also referred to as a quantum physical or quantum mechanical tunnel effect.

In order to use the quantum-mechanical tunnel effect to maintain the tunnel current, an electrical voltage is preferably applied which can range from a few mV (millivolts) to a few V (volts). The measuring tip is preferably located at a distance in the angstrom range ( ^10′1 ° m (meter)) from the microphone membrane during the contactless oscillation to the microphone membrane. Due to the voltage applied and the small distance, a tunnel current flows between the microphone membrane and the measuring tip. The tunnel current essentially depends on the distance, the applied voltage and the work function of the materials used.

The measurable tunneling current is usually accompanied by low currents, which typically range from a few pA (picoamperes) to a few nA (nanoamperes) and sometimes even a few mA (milliamperes). Therefore, even small deviations in the current strength of the tunnel current can be detected particularly clearly as a measurement signal, since the tunnel current depends exponentially on the distance and the smallest changes in the distance lead to a considerable change in the tunnel current as a measurement signal.

In a further preferred embodiment, the MEMS microphone is characterized in that the cantilever oscillates at a frequency of more than 20 kHz (kilohertz), preferably more than 50 kHz, particularly preferably more than 100 kHz. The stated frequencies of the vibration of the cantilever are preferably many times higher than the expected vibrations of the microphone membrane, which result from the impingement of sound waves. The frequency of the oscillation of the cantilever is preferably higher than possible frequencies of the microphone membrane by a factor of at least 2, 3, 4, 5, 10, 15, 20, 50, 100, 200, 500, 1000 or more.

Advantageously, the vibration behavior of the cantilever is independent of the vibration of the microphone membrane due to the specified frequencies of its vibration. This has the advantageous result that the microphone membrane can oscillate in a wide frequency range and, furthermore, a reliable detection of an acoustic signal is provided by the tunnel current. Since it is preferred that the cantilever oscillates at a rate that is many times higher than that of the microphone membrane, it is possible to measure sound events permanently, reliably and with high accuracy. The specified frequencies have proven to be particularly efficient in order to easily and effectively generate and measure a tunnel current between the microphone membrane and the measuring tip. Advantageously, the MEMS microphone according to the invention can be used to precisely record a large frequency range of the microphone membrane, in particular from very low frequencies (<10 Hz) to high frequencies in the kHz or even MHz range.

In a further preferred embodiment, the MEMS microphone is characterized in that the electronic circuit is set up to keep an amplitude and a center position of an oscillation of the measuring tip and/or the cantilever constant, with a change in an amplitude of the tunnel current between the microphone membrane and the measuring tip is measured, with the amplitude of the tunnel current depending on the vibration behavior of the microphone membrane.

By analogy with scanning tunneling microscopy, the embodiment in the last disclosed paragraph corresponds to the so-called constant height mode. In the case of the scanning tunneling microscope, the measuring tip follows a previously specified height profile without the distance between a sample to be examined and the measuring tip of the scanning tunneling microscope having to be readjusted.

The principle of the constant height mode can advantageously be transferred to the MEMS microphone for a detection of sound events according to the invention by a tunnel current. In this case, the electronic circuit is preferably set up in such a way that the amplitude and the center position of the cantilever and/or the measuring tip do not change. A center position of the cantilever and/or the measuring tip preferably designates a spatial position that serves as a reference point for the amplitude of the oscillation. The amplitude of the oscillation is preferably the maximum deflection of the cantilever and/or the maximum displacement of the measuring tip around the middle position. The amplitude, in particular the maximum deflection, can preferably be specified by a variable with the dimension of a distance. The amplitude is preferably set in such a way that, with regard to the expected deflections of the microphone membrane, contact between the microphone membrane and the measuring tip is avoided. In preferred embodiments, the cantilever and thus in particular also the measuring tip oscillates, which cause deflections in the range from pm (picometer) to nm (nanometer) and pm (micrometer).

To illustrate the measurement principle, two cases are considered, namely a theoretical case in which the microphone membrane is not excited to vibrate by sound waves but is essentially stationary, and the actual case in which the microphone membrane is excited to vibrate by sound waves.

In the theoretical case, in which the microphone membrane essentially does not vibrate, there is a fixed distance between the microphone membrane and the center position of the cantilever and/or the measuring tip, provided that the cantilever and thus also the measuring tip oscillate with a constant amplitude around a constant center position executes This means that the tunnel current also has a uniform amplitude, since the distance between the microphone membrane and the measuring tip only changes as part of the oscillation of the cantilever.

Sound waves that hit the microphone membrane through the sound inlet opening cause it to vibrate. Due to the vibrations of the microphone membrane, there is now no fixed distance between the center position of the cantilever and/or the measuring tip and the microphone membrane. In particular, the distance between the microphone membrane and the measuring tip changes depending on the vibration behavior of the microphone membrane. The higher the sound pressure level, the more the microphone membrane is deflected. The more the microphone membrane is deflected, the smaller the distance between the measuring tip and the microphone membrane, so that the tunnel current increases measurably.

Conversely, the lower the sound pressure level of impinging sound waves, the weaker the deflection of the microphone membrane and the lower the tunnel current.

The changes in the signal strength of the tunnel current thus directly reflect the vibration behavior of the microphone diaphragm, which is dependent on the sound waves that hit it.

Advantageously, the measuring principle of a constant height while maintaining a constant amplitude and center position of the oscillation of the cantilever and/or the measuring tip is particularly easy to set up and allows high-frequency oscillation excitation of the cantilever in a robust way.

In a further preferred embodiment, the MEMS microphone is characterized in that the electronic circuit is set up such that an amplitude of the tunnel current between the microphone membrane and the measuring tip is kept constant, with an oscillation of the cantilever and/or the measuring tip being regulated in order to to keep constant a distance between the microphone membrane and a center position of the measuring tip.

By analogy with scanning tunneling microscopy, the embodiment in the last disclosed paragraph corresponds to the so-called constant current mode. The height of the measuring tip is continuously changed to such an extent that the tunnel current remains constant. With the scanning tunneling microscope, this is done via a closed control circuit for controlling the distance between the measuring tip and the sample to be examined.

The principle of the mode of a constant tunneling current from scanning tunneling microscopy can advantageously be transferred to a detection of sound events by a tunneling current according to the invention. It is preferred that the electronic circuit is set up to keep the amplitude of the tunnel current between the microphone membrane and the measuring tip constant. For this purpose, the vibration of the cantilever and/or the measuring tip is regulated in such a way that the distance between a microphone membrane and a middle position of the measuring tip remains constant.

The middle position of the measuring tip designates in particular the reference point for the amplitude of the oscillation of the measuring tip. The center position of the cantilever does not have to be identical to the center position of the measuring tip. The measuring tip is preferably located essentially at the free end of the cantilever and also has a certain spatial extent. Thus, the middle position of the measuring tip means that position with which the amplitude of the oscillation of the measuring tip can be described. Because the cantilever grips the probe tip and oscillates, the probe tip vibrates in the same pattern as the cantilever, i. H. the course of the oscillation is essentially the same.

It is preferred that the amplitude of the tunneling current remains constant by keeping the distance between the center position of the measuring tip and the microphone membrane constant. This is preferably done by means of a closed control loop. The tunnel current is preferably adjusted to a predetermined amplitude by the closed control circuit in the event of an oscillation, in particular a deflection, of the microphone diaphragm. This is preferably done by adjusting the distance between the center position of the measuring tip and the deflection of the microphone membrane. In this case, the closed-loop control circuit is a circuit that can be configured, for example, by the electronic circuitry of the MEMS microphone. The closed control loop performs the tasks of measuring, comparing and adjusting. The tunnel current is thus measured by the closed control circuit, the amplitude of the measured tunnel current is compared with a specified value and adjusted accordingly if there is a deviation. The amplitude of the tunnel current is preferably adjusted by adjusting the center position of the measuring tip. For this purpose, it is preferred that the cantilever (including the measuring tip) performs a translational movement in order to adjust the distance between the measuring tip and the microphone membrane. Such a translational movement of the cantilever is preferably effected by a further actuator. In preferred embodiments, the additional actuator for performing the translational movement of the cantilever can be a MEMS-based drive. For example, a MEMS drive can be a micro-actuator that is connected to the cantilever by coupling elements.

As above, two cases are considered to illustrate the measurement principle, namely an idealized case in which the microphone diaphragm essentially does not oscillate and an actual case in which the microphone diaphragm is excited to oscillate by sound waves. In the theoretical case, in which the microphone membrane essentially does not vibrate, the distance between the middle position of the measuring tip and the microphone membrane does not change. The tunnel current has a constant amplitude. The closed-loop control recognizes that it is not necessary to adjust the distance between the center position of the measuring tip and the microphone membrane, since there is no deviation from a predetermined amplitude of the tunneling current.

If sound waves hit the microphone membrane through the sound inlet opening, it is caused to vibrate. The vibrations of the microphone membrane initially change the distance between the microphone membrane and the middle position of the measuring tip. This also affects the tunnel current, in particular the amplitude of the tunnel current deviates from a predetermined value. A change in the amplitude of the tunnel current is detected by the closed control circuit. In particular, a comparison can be made between the specified and the measured tunnel current through the closed control loop, for example by forming a difference and/or a ratio. In order to then regulate the tunnel current back to the specified value, the distance between the center position of the measuring tip is adjusted in such a way that the specified amplitude of the tunnel current is reached again.

The higher the sound pressure level, the more the microphone membrane is deflected. The more the microphone membrane is deflected, the greater the change in the distance between the center position of the measuring tip and the microphone membrane, which must be adjusted using the controlled variable of a tunnel current that is to be kept constant.

Conversely, the lower the sound pressure level of the incident sound wave, the weaker the deflection of the microphone membrane. The weaker the microphone membrane is deflected, the less the distance between the center position of the measuring tip and the microphone membrane changes, which must be regulated using the measured tunnel current.

Controlling the distance based on the controlled variable of the tunnel current, which is to be kept constant, advantageously allows sound signals to be measured with particularly high sensitivity and accuracy. A closed control circuit also allows very precise tracking of the vibration behavior of the microphone membrane, which not only allows conclusions to be drawn about the frequency and sound pressure level of the sound signals, but also the entire course over time.

In a further preferred embodiment, the MEMS microphone is characterized in that the electronic circuit is set up to apply a bias voltage to the microphone membrane, so that a zero point position and/or the ability to oscillate the microphone membrane can be regulated.

Advantageously, the sensitivity of the microphone membrane can be regulated and/or adjusted by applying a bias voltage to the microphone membrane. In this case, a bias voltage within the meaning of the invention refers to an electrical voltage that is applied so that the microphone membrane achieves a curvature that protrudes beyond a zero point position of the microphone membrane. The zero point position of the microphone membrane preferably denotes the force-free resting position of the Microphone membrane when there is no deflection by sound waves and no bias is applied. Thus, by applying a bias voltage, the positioning of the microphone membrane can deviate from the zero point position and/or result in the microphone membrane having a curvature that deviates from the zero point position. It is preferred that the bias voltage can be regulated by the electronic circuit.

Advantageously, the application of a bias allows the sensitivity of the microphone membrane to be regulated. If a bias voltage is applied, this affects the vibrating ability of the microphone membrane. The higher the bias voltage applied, the lower the ability of the microphone membrane to oscillate. The lower the applied bias voltage, the higher the ability of the microphone diaphragm to vibrate. In this way, the sensitivity of the microphone membrane in particular can advantageously be precisely adjusted.

The sensitivity of the microphone diaphragm preferably designates the ability to experience a deflection, with the deflection being dependent on the sound pressure levels of the sound waves entering through the sound entry opening.

Regulating the ability of the microphone membrane to oscillate is advantageous in that, particularly with regard to the small distances when a tunnel current occurs, an optimal adjustment of the MEMS microphone to different sound pressure levels can be ensured. The MEMS microphone according to the invention thus has high dynamics and is advantageously suitable for a wide range of sound signals to be measured.

In a further preferred embodiment, the MEMS microphone is characterized in that the MEMS microphone has a sensitivity which allows sound pressure waves with a sound pressure level of less than 20 dB, particularly preferably less than 10 dB, less than 5 dB, less than 1 dB or less than 0 dB to measure.

Advantageously, the MEMS microphone according to the invention can measure low sound pressure levels, as just indicated, with extreme precision. The sound pressure level (English sound pressure level, abbreviated to SPL) is the decadic logarithm of the squared relationship between the effective value of the measured sound pressure and its reference value of 20 pPa (micropascals) commonly used in acoustics. In this way, sound pressure levels with a high signal-to-noise ratio can advantageously be measured.

It is also advantageous that, despite the low sound pressure level, sound events can be measured with high resolution using the compact MEMS microphone according to the invention. In this case, the lateral resolution preferably designates a resolution perpendicular to the course of a measurement path by means of sound waves. The opposite of the lateral resolution is the axial resolution along the length of the measurement path, i.e. the path of the sound.

In particular, the lateral resolution is the distance between two adjacent objects, e.g. B. two sound sources that can be mapped as two points. This advantageously makes it possible to create a very precise image of a sound field, despite the low sound pressure level.

In addition, low sound pressure levels are particularly relevant in the context of photoacoustic spectroscopy, so that the MEMS microphone according to the invention is preferably used photoacoustic measurements can be used efficiently. Other advantageous uses of the MEMS microphone according to the invention are also possible. Low sound pressure levels can also be caused by machines such as B. lighting fittings arise. The MEMS microphone according to the invention is therefore advantageously suitable for monitoring devices in which low sound pressure levels are relevant. Advantageously, the MEMS microphone can be used optimally and efficiently in a large number of possible applications.

In a further preferred embodiment, the MEMS microphone is characterized in that the electronic circuit is set up so that the actuator regulates the oscillation of the cantilever in such a way that there is a distance of between 0.1 nm and 100 nm is present. Intermediate values such as maximum distances between 1 nm and 50 nm or 0.5 nm and 10 nm can also be preferred.

The distance between the measuring tip and the microphone membrane is regulated in particular by the center position of the vibration of the cantilever and/or the amplitude of the vibration excitation. The specified distance range has proven to be particularly advantageous, on the one hand to be able to measure the vibration behavior of the membrane with high resolution using the tunnel current and on the other hand to effectively avoid contact between the measuring tip and the membrane. The MEMS microphone according to the invention is therefore distinguished by a long-term stable measurement capability.

In a further preferred embodiment, the MEMS microphone is characterized in that the actuator excites the cantilever comprising the measuring tip to oscillate, with the actuator preferably being selected from a group comprising a piezoelectric actuator, an electrostatic actuator, an electromagnetic actuator, a magnetostrictive actuator and/or a thermal actuator.

The actuators mentioned above are particularly well suited for exciting a large number of rapid oscillations and have a low energy requirement, in particular due to the compact design. The bandwidth of the achievable vibrations is also advantageously high due to the compact design and the low inertia.

Preferably the actuator is a MEMS actuator. A MEMS actuator is preferably an actuator that is produced using standard production methods of microsystems technology and also has dimensions in the order of pm (micrometers). Such an actuator is particularly compact, robust and low-maintenance and can be produced easily and inexpensively. In particular, the cantilever, which is excited to oscillate by the actuator, can also be a MEMS element. This preferably means that the cantilever and the actuator can preferably be produced in one production step with the MEMS actuator and are compact. Desirably, parts of the same substrate can be used for production. This simplifies and makes production considerably cheaper. A piezoelectric actuator preferably refers to an actuator that uses the piezoelectric effect. In particular, a piezoelectric actuator can stimulate the cantilever to oscillate using the inverse piezoelectric effect. The piezoelectric effect includes the direct piezoelectric effect, which describes the occurrence of an electrical voltage when certain solid bodies, in particular piezoelectric crystals, are deformed, and the inverse piezoelectric effect, in which a deformation is brought about by applying an electrical voltage. A piezoelectric actuator preferably comprises a piezoelectric crystal, which deforms when an electrical voltage is applied. Depending on the piezo crystal and the cut, it becomes longer, wider and/or bends.

An electrostatic actuator preferably uses electrostatic fields to move components, particularly the cantilever. For this purpose, the cantilever preferably has a material that reacts to the electrostatic fields used. An electromagnetic actuator converts electrical energy into mechanical energy. The effects of electromagnetism are preferably used here. In the case of a thermal actuator, a heat source is preferably used in order in particular to generate movements of the cantilever.

Magnetostrictive actuators are preferably based on the change in length of ferromagnetic materials. Magnetostrictive actuators are preferably made by sintering techniques and change length under magnetic fields. These actuators can also be used advantageously under high pressures and temperatures as control elements with high positioning accuracy in the micrometer range.

In a further preferred embodiment, the MEMS microphone is characterized in that the oscillatable microphone membrane comprises an electrically conductive material, the electrically conductive material preferably being selected from a group comprising monosilicon, polysilicon, molybdenum, tantalum, aluminum, graphite, tungsten, titanium , platinum, gold, palladium, iron, copper, silver, brass, chromium, their compounds and/or alloys, with the oscillatable microphone membrane optionally comprising an additional non-electrically conductive material, which is preferably selected from a group consisting of silicon nitride and/or silicon dioxide .

These materials are easy and inexpensive to process in semiconductor and/or microsystems fabrication and lend themselves to large-scale fabrication. Due to the materials and/or manufacturing methods, the microphone membrane can advantageously be manufactured flexibly. In particular, it is preferably possible to manufacture the MEMS microphone comprising the oscillatable microphone membrane together with a carrier in a (semiconductor) process, preferably on a substrate. This further simplifies and reduces the cost of manufacture, so that a compact and robust MEMS microphone can be provided at low cost.

In addition, the materials listed are advantageous in that they are characterized by particularly inert behavior. Because they are inert, they do not react with the environment in which the MEMS microphone according to the invention is to be installed, so that a particularly robust and insensitive MEMS microphone can advantageously be provided. Electrically conductive material is preferably used in this case, in order in particular to ensure that the tunnel current is maintained.

It is also preferred to provide the microphone membrane with semiconducting and/or dielectric materials. Dielectric materials preferably mean electrically non-conductive materials. In particular, dielectric materials are preferably introduced into the microphone membrane in such a way that they are embedded within the microphone membrane. Advantageously, the preferably used semiconducting and/or dielectric materials in the microphone membrane result in the microphone membrane being supported advantageously mechanically. The sensitivity and thus the ability to oscillate the microphone membrane can also be advantageously adjusted in this way.

In a further preferred embodiment, the MEMS microphone is characterized in that the cantilever and/or the measuring tip comprises a material selected from a group comprising silicon, iridium, tungsten, platinum, palladium and/or gold.

These materials advantageously have the desired electrical, mechanical and/or thermal properties in order to allow a tunnel current to flow particularly efficiently between the microphone membrane and the measuring tip. Furthermore, the materials can be processed very easily and inexpensively in order to provide the cantilever and/or the measuring tip.

The tunnel current is particularly relevant with regard to the provision of the measuring tip in terms of dimensioning. In particular, the probe tip can be designed in such a way that one atom is the foremost tip of the probe tip and/or is responsible for most of the tunneling current.

In a further preferred embodiment, the MEMS microphone is characterized in that the measuring tip has a radius of up to 15 nm, preferably up to 10 nm, particularly preferably up to 5 nm.

In a further preferred embodiment, the MEMS microphone is characterized in that the cantilever has a length of up to 1000 μm, a width of up to 100 μm and a thickness of up to 10 μm.

The listed dimensions of the cantilever and/or the measuring tip have proven to be advantageous in that on the one hand they enable a tunnel current between the microphone membrane and the measuring tip in a particularly reliable manner and on the other hand they are designed to be particularly optimally capable of oscillating, so that the guidance to the microphone membrane can be carried out particularly efficiently, for example through the actuator.

The average person skilled in the art recognizes that technical features, definitions and advantages of preferred embodiments which apply to the MEMS microphone according to the invention for detecting acoustic signals also apply to a method for detecting acoustic signals comprising the MEMS microphone according to the invention and vice versa.

In a further aspect, the invention relates to a method for detecting acoustic signals, comprising a MEMS microphone comprising a sound entry opening, an oscillatable one Microphone membrane and an electronic circuit, with sound waves entering through the sound entry opening exciting the oscillatable microphone membrane to oscillate, characterized in that the MEMS microphone has a cantilever comprising a measuring tip and an actuator, the cantilever and/or the measuring tip being actively closed by the actuator Oscillations are stimulated and guided to the microphone membrane in a non-contact oscillating manner, so that a tunnel current is measured between the measuring tip and the microphone membrane and the tunnel current allows detection of the vibration behavior of the microphone membrane, which is dependent on the sound waves.

The method for detecting acoustic signals comprising the MEMS microphone according to the invention advantageously allows high-resolution measurement of low sound pressure levels. Sound pressure levels can thus be measured which are in particular less than 20 dB (decibels), preferably less than 10 dB, particularly preferably less than 5 dB, very particularly preferably less than 1 dB. It is particularly advantageous that the low sound pressure levels can be measured with a particularly high signal-to-noise ratio.

The average expert knows that the signal-to-noise ratio describes the ratio of the actual signal component to the noise component. By specifying in dB, the signal-to-noise ratio can be better quantified. Noise is any interference that can affect the signals. The reception quality of the recorded signals can thus be evaluated using the signal-to-noise ratio. The greater the signal-to-noise ratio, the smaller the noise component compared to the useful or measurement signal and the easier it can be filtered out. Especially in the context of measuring low sound pressure levels, the signal-to-noise ratio was not high in the prior art, since the influence of noise is weighted higher. It is therefore to be rated as particularly advantageous that the MEMS microphone according to the invention can also measure at low sound pressure levels with a comparatively high signal-to-noise ratio.

By measuring with the aid of a tunnel current, an extremely sensitive measurement is advantageously possible, since even the slightest deviations are clearly recognizable, in particular due to the low dimensioning of the current strength of the tunnel current.

At the same time, contact between the measuring tip and the microphone membrane can advantageously be avoided, so that a reliable, long-term stable and safe measurement of the vibration behavior of the microphone membrane and thus of the sound waves that occur is made possible.

In addition, the measurement can advantageously be carried out without flow losses of the fluid in which the sound propagates, so that a particularly precise image of the sound field is made possible.

In a further aspect, the invention relates to the use of the MEMS microphone according to the invention or preferred embodiments thereof for photoacoustic spectroscopy.

In a further aspect, the invention relates to a photoacoustic gas sensor comprising a modulatable emitter, an analysis volume that can be filled with gas and a MEMS microphone according to the invention or a preferred embodiment thereof, wherein the modulatable emitter and the MEMS microphone are arranged in such a way that the emitter can excite gas in the analysis volume to form sound pressure waves by means of modulatably emitable radiation, which with the aid of the MEMS sensor can be detected.

The average person skilled in the art recognizes that technical features, definitions and advantages of preferred embodiments of the MEMS microphone according to the invention also apply to the method or use according to the invention and to a photoacoustic gas sensor comprising a MEMS microphone described and vice versa.

A photoacoustic gas sensor is known to a person skilled in the art in terms of its basic features and essential components. A modulatable emitter generates electromagnetic radiation and is preferably arranged and configured in such a way that the radiation emitted by the infrared emitter essentially or at least partially impinges on the gas in the analysis volume.

If the modulated irradiation occurs with a wavelength which corresponds to the absorption spectrum of a molecule of a gas component present in the gas mixture, modulated absorption takes place, which leads to heating and cooling processes whose time scales reflect the modulation frequency of the radiation. According to the photoacoustic effect, the heating and cooling processes lead to expansion and contraction of the gas component, which stimulates the formation of sound pressure waves with essentially the modulation frequency. The sound pressure waves are also referred to as PAS signals and can be measured particularly sensitively using the MEMS microphone as described. The power of the sound waves is preferably directly proportional to the concentration of the absorbing gas component. Due to the possibility of measuring extremely low sound level pressures using the MEMS microphone according to the invention, even the smallest proportions of gas components can advantageously be detected.

Various emitters are preferred as radiation sources for the applications mentioned. For example, narrow-band laser sources can be used. These advantageously allow the use of high radiation intensities and can preferably be modulated at high frequency using standard components for photoacoustic spectroscopy. Broadband emitters can also preferably be used. These advantageously have a wide range, which z. B. can be further selected by using (tunable) filters. The emitter that can be modulated is preferably a modulatable infrared emitter.

In one embodiment, the modulatable emitter can be a thermal emitter comprising a heating element, the heating element comprising a substrate on which a heatable layer made of a conductive material is at least partially applied, on which contacts for a current and/or voltage source are present. The heating element includes a Heatable layer made of a conductive material that produces Joule heat when an electric current flows through it. In particular, the heating element comprises a substrate on which the heatable layer is present. The substrate preferably forms the base of the heating element. In this case, the substrate can also at least partially include other elements of the IR emitter, such as for example a base element and/or housing elements.

The emitter can be modulated, which means that the intensity of the emitted radiation, preferably the intensity of the beam, can be changed in a controllable manner over time. The modulation should preferably bring about a change in the intensity over time as a measurable variable. That means e.g. B. that the intensity over time between the weakest intensity measured within the measurement period and the strongest intensity measured within the same period of time there is a difference that is greater than the sensitivity of a device typically used for the radiation spectrum and the application for measuring or determining the Intensity. The difference is preferably significantly greater than a factor of 2, more preferably 4, 6 or 8 between the strongest and the weakest adjustable intensity. Particularly preferably, the intensity of the modulated beam is modulated for one or more predetermined resonance wavelengths.

Direct modulation can preferably be carried out by varying the power supply. In the case of a thermal emitter, such a modulation is usually limited to a certain range of a modulation spectrum, e.g.

B. in the range of an order of up to 100 Hz. B. a laser or an LED are preferably significantly higher modulation rates, z. B. in the kHz range and beyond, possible.

The emitter can preferably also be modulated by external modulation, e.g. B. through the use of a rotating chopper wheel and / or an electro-optical modulator.

The gas to be analyzed is preferably located in an analysis volume that can be filled with gas. In a preferred embodiment, this is a volume (or chamber) that is at least partially closed or lockable to the outside, in which the gas is located or can be introduced, e.g. B. through a closable opening in the form of a closure and / or valve and / or through a supply line. However, it can also be a completely closed or lockable volume or chamber, which has at least one, preferably two, lockable openings for introducing and/or discharging the gas to be analyzed. The gas to be analyzed can thus be localized very well, in particular in a beam area of the emitter, for example infrared radiation.

The analysis volume can preferably also be at least partially open. In this way, in particular, a gas atmosphere surrounding the spectroscope, to which the analysis volume is at least partially open, can be measured and checked for its composition. This is particularly interesting for applications in the field of pollutant measurement, but also z. B. for military applications or counter-terrorism, z. B. by a poison gas attack. In this case, it is advantageous that the analysis volume is well defined, so that the emitter, the analysis volume and the MEMS microphone are arranged in such a way that the radiation that can be emitted by the emitter in a modulable manner can excite gas in the analysis volume to form sound pressure waves, which can be detected with the help of the MEMS microphones are measurable.

The analysis volume is preferably in the beam path of the emitter. This preferably means that the intensity of the beam essentially or at least partially impinges on the side of the analysis volume facing the emitter. Partially means preferably at least 40%, preferably at least 50%, 60%, 70%, 80% or more.

An analysis volume can be formed by a chamber. However, it can also be preferred that the analysis volume comprises a sample chamber and a reference chamber, which are connected or can be connected by a connecting channel.

In the case of an embodiment of an analysis volume, which has a sample chamber and a reference chamber, it may be preferable to position a MEMS microphone in each chamber in order to measure separately in each chamber and thus eliminate sources of interference, e.g. B. external sound pressure waves, which do not originate from the radiation absorbed in the sample chamber, preferably to be able to calculate them out after the measurement.

It can also be preferred that the emitter irradiates the sample chamber and not the reference chamber and that there is a connecting channel between the sample chamber and the reference chamber, in which the MEMS microphone is located as a sound detector. The same gas can be found in the sample volume and reference volume. It can also be preferred that different gas is contained in the sample volume and in the reference volume, a gas with known properties being present in the reference volume and a gas to be analyzed being present in the sample volume. This embodiment is distinguished by a particularly precise photoacoustic spectroscopy, since, for example, sound from undesired sound sources is eliminated or not included in the measurement and/or the evaluation of the measurement. Sample volumes and a reference volume can preferably have essentially the same dimensions in order to implement an accurate differential measurement method.

The MEMS microphone according to the invention is to be explained in more detail below using examples, without being restricted to these examples.

CHARACTERS

Brief description of the characters

Fig. 1 Schematic representation of a preferred MEMS microphone

Detailed description of the figures

1 shows a schematic representation of a preferred MEMS microphone 1. As explained in the above statements, the MEMS microphone 1 is particularly well suited to measuring low sound pressure levels with high resolution. The MEMS microphone 1 includes an oscillatable microphone membrane 3. If a sound event occurs, sound waves 5 propagate from a sound source in the direction of the MEMS microphone 1 through a sound inlet opening and hit the microphone membrane 3, which is then excited to oscillate. Furthermore, the MEMS microphone 1 has a cantilever 7 including a measuring tip 9 . Other components such as an electronic circuit and an actuator are not shown in FIG.

The electronic circuit is set up in such a way that the cantilever 7 including the measuring tip 9 is actively excited to oscillate, it being possible for the transmission to be carried out by the actuator to oscillate. The oscillation of the cantilever 7 and thus of the measuring tip 9 is intended to be illustrated by the dashed line and the curved arrow below the measuring tip 9. The measuring tip 9 and the microphone membrane 3 comprise electrically conductive material. The measuring tip 9 is brought so close to the microphone membrane 3 that it oscillates without making contact, so that a tunnel current (not shown) flows based on the quantum-mechanical tunnel effect. The tunnel current reflects the vibration of the measuring tip 9 as a periodic signal, since the distance between the measuring tip 9 and the microphone membrane changes periodically. The measurable tunnel current allows detection of the vibration behavior of the microphone membrane 3, which is caused by the sound waves 5, in particular of sizes such. B. the sound pressure level and / or the frequency depends.

The MEMS microphone 1 can advantageously be used to measure low sound pressure levels with a simultaneously high signal-to-noise ratio, since the tunnel current can measure the smallest deflections of the microphone membrane 3 particularly precisely due to the exponential dependence on the distance.

The MEMS microphone 1 is also advantageous in that contact between the measuring tip 9 and the microphone membrane 3 is avoided in a particularly reliable manner. Furthermore, there are advantageously no flow losses of the sound, so that a particularly precise sound image can be generated. The MEMS microphone 1 can thus measure sound pressure levels with high precision and reliability and can precisely determine even the slightest deviations.

The cantilever 7 and thus also the measuring tip 9 perform vibrations that are significantly faster than the expected vibrations of the oscillatable microphone membrane 3, so that advantageously the vibration of the cantilever 7 and/or the measuring tip 9 is independent of the vibration of the microphone membrane 3. Therefore, the microphone membrane 3 can oscillate in a wide frequency range while at the same time providing a reliable and particularly sensitive possibility of detecting sound waves through the tunnel current.

The MEMS microphone 1 can have different operating modes.

The electronic circuit can be set up to keep an amplitude and a center position of an oscillation of the measuring tip 9 and/or of the cantilever 7 constant. A change in the amplitude of the tunnel current is measured, which provides information about the vibration behavior of the microphone membrane 3 and about sound parameters such as the sound pressure level of sound waves 5 incident. Furthermore, the electronic circuit can be configured in such a way that an amplitude of the tunnel current between the microphone membrane 3 and the measuring tip 9 is kept constant. This is achieved in that an oscillation of the cantilever 7 and thus of the measuring tip 9 is regulated in such a way that a distance between the microphone membrane 3 and a central position of the measuring tip 9 is kept constant. This is done in particular by a closed

Control loop, which can be given for example by the electronic circuit. In this case, the necessary adjustment of the distance between the microphone membrane 3 and a central position of the measuring tip 9 allows direct conclusions to be drawn about the vibration behavior of the membrane and therefore the sound waves 5 that are impinging.

ti REFERENCE LIST

1 MEMS microphone

3 Vibrating Microphone Diaphragm

5 sound waves

7 cantilevers

9 measuring tip

BIBLIOGRAPHY

Sievilä, Päivi, et al. "Sensitivity-improved silicon cantilever microphone for acousto-optical detection." Sensors and Actuators A: Physical 190 (2013): 90-95.

Claims

PATENT CLAIMS 1 . MEMS microphone (1) for detecting acoustic signals, comprising a sound entry opening, an oscillatable microphone membrane (3) and an electronic circuit, with sound waves (5) entering through the sound entry opening exciting the oscillatable microphone membrane (3) to oscillate, characterized in that the MEMS - Microphone (1) has a cantilever (7) comprising a measuring tip (9) and an actuator, the electronic circuit for measuring a tunnel current between the microphone membrane (3) and the measuring tip (9) and for active excitation of the cantilever ( 7) is set up to vibrate, with the measuring tip (9) being guided to the microphone membrane (3) in a non-contact vibrating manner to detect acoustic signals, while the measurable tunnel current allows detection of the vibration behavior of the microphone membrane (3), which is dependent on the sound waves (5). 2. MEMS microphone (1) according to the preceding claim, characterized in that the cantilever (7) oscillates at a frequency of more than 20 kHz, preferably more than 50 kHz, particularly preferably more than 100 kHz. 3. MEMS microphone (1) according to one or more of the preceding claims, characterized in that the electronic circuit is set up to keep an amplitude and a center position of an oscillation of the measuring tip (9) and/or the cantilever (7) constant , wherein a change in an amplitude of the tunnel current between the microphone membrane (3) and the measuring tip (9) is measured, the amplitude of the tunnel current depending on the vibration behavior of the microphone membrane (3). 4. MEMS microphone (1) according to one or more of the preceding claims, characterized in that the electronic circuit is set up so that an amplitude of the tunnel current between the microphone membrane (3) and the measuring tip (9) is kept constant, with an oscillation of the cantilever (7) and/or the measuring tip (9) is regulated in order to keep a distance between the microphone membrane (3) and a middle position of the measuring tip (9) constant. 5. MEMS microphone (1) according to one or more of the preceding claims, characterized in that the electronic circuit is set up to apply a bias voltage to the microphone membrane (3), so that a zero point position and/or the ability to oscillate the microphone membrane (3) can be regulated can. MEMS microphone (1) according to one or more of the preceding claims, characterized in that the MEMS microphone (1) has a sensitivity which allows sound pressure waves (5) with a sound pressure level of less than 20 dB, particularly preferably less than 10 dB to measure. MEMS microphone (1) according to one or more of the preceding claims, characterized in that the electronic circuit is set up so that the actuator regulates the vibration of the cantilever (7) in such a way that between maximum deflections of the measuring tip (9) and the oscillatable microphone membrane (3) there is a distance between 0.1 nm and 100 nm. MEMS microphone (1) according to one or more of the preceding claims, characterized in that the actuator excites the cantilever (3) comprising the measuring tip (9) to oscillate, the actuator preferably being selected from a group comprising a piezoelectric actuator, an electrostatic actuator, an electromagnetic actuator and/or a thermal actuator. MEMS microphone (1) according to one or more of the preceding claims, characterized in that the oscillatable microphone membrane (3) comprises an electrically conductive material, the electrically conductive material preferably being selected from a group comprising monosilicon, polysilicon, molybdenum, tantalum, aluminum , Graphite, tungsten, titanium, platinum, gold, palladium, iron, copper, silver, brass, chromium, their compounds and/or alloys, with the oscillatable microphone membrane (3) optionally comprising an additional non-electrically conductive material, which is preferably selected from a group comprising silicon nitride and/or silicon dioxide. MEMS microphone (1) according to one or more of the preceding claims, characterized in that the cantilever (7) and/or the measuring tip (9) comprises a material selected from a group comprising silicon, iridium, tungsten, platinum, palladium and/or or gold. MEMS microphone (1) according to one or more of the preceding claims, characterized in that the measuring tip (9) has a radius of up to 15 nm, preferably up to 10 nm, particularly preferably up to 5 nm. MEMS microphone (1) according to one or more of the preceding claims, characterized in that the cantilever (7) has a length of up to 1000 μm, a width of up to 100 μm and a thickness of up to 10 μm. Method for detecting acoustic signals comprising a MEMS microphone (1) comprising a sound entry opening, an oscillatable microphone membrane (3) and an electronic circuit, wherein sound waves (5) entering through the sound entry opening excite the oscillatable microphone membrane (3) to oscillate, characterized in that that the MEMS microphone (1) has a cantilever (7) comprising a measuring tip (9) and an actuator, the cantilever (7) and/or the measuring tip (9) being actively excited to oscillate by the actuator and oscillating without contact the microphone membrane is guided, so that a tunnel current is measured between the measuring tip (9) and the microphone membrane (3) and the tunnel current allows detection of the sound waves (5) dependent vibration behavior of the microphone membrane (3). Use of the MEMS microphone (1) according to one or more of Claims 1 - 12 for photoacoustic spectroscopy and/or infrared spectroscopy. Photoacoustic gas sensor comprising a modulatable emitter, an analysis volume that can be filled with gas and a MEMS microphone (1) according to one of the preceding claims -13, wherein the modulatable emitter and the MEMS microphone are arranged in such a way that the emitter uses modulatably emissible radiation gas in can stimulate the analysis volume to form sound pressure waves, which can be detected using the MEMS sensor.